drivers/nrf24l01+ : added netdev implementation for nrf24l01+ transceiver

This PR is based on #1704. It contains a connection between the lowlevel part for the nrf24l01+ transceiver-driver and the netdev interface. I assume there will be several deficits, especially because this device fulfills no standard. So let's discuss this!
If a user sets the option with a 1 byte (or 2 byte, 3 byte maybe but this is ridiculous :-D) number this should be excepted. If the address is 4 byte long in fact only the lowest byte is set.